Over the past few years, extensive research has been carried out on enhancing the performance of lithium ion secondary batteries, the aim being to increase energy density and safety. In particular, given that the active material serving as the electrical energy-storing core has a direct bearing on enhanced performance in lithium ion secondary batteries, intensive research and development is being conducted on materials which, in terms of electrical capacity and other characteristics, are superior to the lithium cobaltate in current use. For example, research is being carried out on inorganic solid active materials such as cobalt-nickel-manganese ternary oxides having higher capacities and energy densities than lithium cobaltate, and olivine-type phosphates that exhibit extremely stable redox behaviors.
At the same time, a number of attempts are being made to use organic substances as active materials. Resources for organic active materials are more abundant than those for existing inorganic layered compounds, and there is a possibility that higher capacity can be achieved by molecular design. For example, among low-molecular-weight π-conjugated compounds such as those mentioned in Patent Documents 1 and 2 are several which exhibit a stable redox behavior involving two or more electrons and whose use as positive electrode materials for lithium ion secondary batteries is being investigated.
However, because low-molecular-weight materials undergo large changes in polarity when electron transfer is carried out, dissolution of these materials in the electrolyte solution readily arises, as a result of which the cycle performance is often low. Also, because these materials lack electrical conductivity, it is necessary to use a large amount of conductive additive in order to lower the internal resistance of the battery. As a result, even if the low-molecular-weight material itself has a large capacity, the capacity of the overall electrode ends up being a small value.
By contrast, main chain-conjugated polymers are materials which can be reversibly oxidized and reduced. Given also the advantage that, as polymers, dissolution in the electrolyte solution can be suppressed, as mentioned in Non-Patent Document 1, practical research is being carried out on the use of such polymers as positive electrode active materials for lithium ion secondary batteries.
However, main chain-conjugated polymers have the drawback that, as oxidation proceeds, electrostatic repulsions between oxidants (radical cations) on the main chain increase, as a result of which the capacity does not reach the theoretical capacity calculated from the structure and ends up being relatively low.
From this perspective, a number of development efforts are being carried out on the use of, as electrode active materials, polymeric compounds which have electrochemically active sites in a non-conjugated form such as that mentioned in Non-Patent Document 2. However, carbazole-containing polymers with a stable oxidation state have not hitherto been employed.